[clang] [clang] Speed up Decl::getASTContext() (PR #226612)

[Hans Wennborg via cfe-commits]

https://github.com/zmodem approved this pull request.

lgtm

But as discussed on the other PR, longer term it would be nice if we could get rid of the pointers from AST to ASTContext since it seems redundant.

https://github.com/llvm/llvm-project/pull/226612